Do not require postBuild to be executable

This is hard for windows users and from the GitHub UI,
and provides dubious benefits. So let's remove the requirement!

Fixes #240

repo2docker/buildpacks/base.py

@@ -3,7 +3,6 @@ import jinja2
 import tarfile
 import io
 import os
-import stat
 import re
 import logging
 import docker
@@ -107,15 +106,14 @@ LABEL {{k}}={{v}}
 # We always want containers to run as non-root
 USER ${NB_USER}
 
+# Make sure that postBuild scripts are marked executable before executing them
 {% if post_build_scripts -%}
 {% for s in post_build_scripts -%}
-RUN ./{{ s }}
+RUN chmod +x {{ s }} && ./{{ s }}
 {% endfor %}
 {% endif -%}
 """
 
-DOC_URL = "http://repo2docker.readthedocs.io/en/latest/samples.html"
-
 
 class BuildPack:
     """
@@ -409,9 +407,5 @@ class BaseImage(BuildPack):
     def get_post_build_scripts(self):
         post_build = self.binder_path('postBuild')
         if os.path.exists(post_build):
-            if not stat.S_IXUSR & os.stat(post_build).st_mode:
-                raise ValueError("%s is not executable, see %s for help." % (
-                                 post_build,
-                                 DOC_URL+'#system-post-build-scripts'))
             return [post_build]
         return []

tests/venv/postBuild/README.rst

@@ -4,16 +4,4 @@ System - Post-build scripts
 It is possible to run scripts after you've built the environment specified in
 your other files. This could be used to, for example, download data or run
 some configuration scripts. For example, this will download and install a
-Jupyter extension.
-
-.. note::
-
-   This file needs to be executable in order to work with ``repo2docker``. If
-   you're on Linux or macOS, run::
-
-       chmod +x postBuild
-
-   If you're on windows, you can accomplish the same behavior with this
-   ``git`` command::
-
-       git update-index --chmod=+x postBuild
+Jupyter extension.